Unsharpness Calculations and

Resolution

By Professor Stelmark

Source-to-object distance (SOD)

refers to the distance from the x-ray source (focal spot) to the object being radiographed. SOD can be expressed mathematically as follows:

OID = The distance between the object (area of interest) and the image receptor

SOD = The distance between the focal spot (source) and object (area of interest)

SOD = SID−OID

Focal spot size FSS = Dimensions of the nominal focal spot in millimeters (mm)

Image 1 Image 2

Focal spot size=0.6 mm SID=40 inches OID=0.25 inch

Focal spot size=1.2 mm SID=56 inches OID=4.0 inches

Geometric unsharpness of Image 1=0.004 mm

Geometric unsharpness of Image 2=0.09 mm

Image 2 has the greater amount of unsharpness.

Calculating Geometric Unsharpness

The amount of geometric unsharpness can be calculated for each of the following images to determine which image has more geometric unsharpness.

                 

SPATIAL RESOLUTION CONTRAST RESOLUTION

NOISE

IMAGE ARTIFACTS

RADIATIONDOSE

TEMPORAL RESOLUTION

SPATIAL RESOLUTION

The degree of sharpness in the image

The degree of blurring in the image

Spatial resolution (also known as high-contrast resolution) is the ability of an imaging system to create separate images of closely spaced high-contrast (black and white) objects (as with contrast resolution, spatial resolution also applies to all imaging modalities). In other words, do the two objects appear sharp and clear, or do they blur together?

The spatial resolution is specified in terms of line pairs per centimeter (lp/cm) or line pairs per millimeter (lp/mm). A line pair is a pair of equal-sized black-white bars. The number of line pairs in digital imaging is specified as spatial frequency.

7 lp/mm 11 lp/mm

11 lp/mm17 lp/mm

The most common method of measuring spatial resolution is to use a value known as spatial frequency. The unit of spatial frequency is the line pairs per millimeter (lp/mm) and is obtained with a resolution chart (Fig. 3-13). A line pair includes an opaque line and a radiolucent space. In the resolution chart, one lp/mm would have a 0.5 mm lead bar separated by 0.5 mm of radiolucent material. Two lp/mm would have 0.25 mm lead bars separated by 0.25 mm of radiolucent material and so the greater the lp/mm value, the smaller the object that can be imaged and the better the spatial resolution. The limiting spatial resolution (also known as the Nyquist frequency) is the maximum number of lp/mm that can be recorded by the imaging system.

The resolving power of the unaided human eye is approximately 30 lp/mm when inspecting an image up close, and at normal reading distance (about 25 cm), it is about 5 lp/mm.

Imaging System Limited Spatial Resolution Smallest Object

Resolved

Analog non screen film system 50 – 100 lp/mm

Analog screen film system 6 -10 lp/mm

Computed Radiography (CR) 2-5 lp/mm

Digital Radiography (DR) 8-10 lp/mm

SPATIAL FREQUENCY- 15 lp/mm. WHAT IS THE SMALLEST SIZE OF AN OBJECT

THE IMAGING SYSTEM CAN RESOLVE

1/15 = 0.06 mm/lp0.06 /2 = 0.03 mm object

Spatial Resolution Influencing Factors in Digital Imaging

• FOV• Matrix size• Type of Algorithm

Contrast Resolution

The ability of an imaging system to distinguish structures with similar x-ray transmission as separate entities (the term not only applies to intensifying screens, but all imaging systems including computed radiography (CR), digital radiography (DR), conventional and digital fluoroscopy, computed tomography, magnetic resonance imaging, and sonography).

Contrast Resolution Influencing Factors in Digital Imaging

• SNR• Dynamic Range• Pixel Depth• Type of Algorithm

TEMPORAL RESOLUTION

Temporal resolution is an indication of a X-ray system's ability to freeze motions of the exposed object. An oversimplified analogy is the “shutter” speed of a camera. When a photo is taken at a sports event, a higher shutter speed should be used to reduce the blurring effects caused by the moving athletes.